The complexity of semiconductor chips has increased dramatically over the past several years. Such increased complexity has lead to an increase in the number of input and output leads or contacts required for each chip. Furthermore, the demand for a small "footprint," i.e., the amount of space required by a component on a printed circuit board, has lead to smaller and smaller package sizes.
In response to these demands, new semiconductor packages (also known as "chip carriers") have been developed that are small in size, yet have a large number of input and output leads or contacts. These packages include, for example, ball grid arrays (BGAs) which are slightly larger than the semiconductor die they package and having contacts (i.e., balls or bumps of solder) that are around 30 mils in diameter. Also, chip size packages (CSPs) which are about the size of the semiconductor die they package and having contacts that are around 10-15 mils in diameter. Additionally, "flip chips" have been developed wherein contacts are adhered directly to the semiconductor wafer, the contacts being approximately 5-10 mils in diameter.
The reduction in the diameter of contacts, and also the general shape of the new contacts, i.e., ball or bump, has made it increasingly difficult to accurately control the quality of the contacts on the semiconductor. In particular, there is a need to accurately measure the dimensions and location of the ball or bump contacts in order to compare the manufactured semiconductor chip (packaged or otherwise) to a manufacturer's specification.
One typical way of measuring the dimensions of contacts or leads on chip carriers is through the use of a three dimensional (3-D) vision system that employs optical triangulation techniques. U.S. Pat. No. 5,465,252 issued to Bilodeau et al. (the "'152 patent"), expressly incorporated herein by reference, describes one such system. A light source such as a laser is positioned to illuminate an object (such as a packaged semiconductor chip) at a specific X-Y position. The source laser beam is directed through an optical system and forms a focused spot at the point of impingement on the object. The focused spot is reflected and light reflected off-axially with respect to the source laser beam is focussed on a light sensor. The image location (i.e., the point where the reflected spot impinges the light sensor) is related, using standard optics principals, to the location of the light spot on the object, which, in turn, can be used to determine image height (Z location). Calibrated processing electronics are used to calculate the height of the object and store the object height with its associated X-Y location. The light source is then positioned to different X-Y locations, and the process is repeated until 3-D data is gathered for the entire object (or a portion thereof). The stored data is then compared to a manufacturer's specification for the object (or portion thereof) to determine whether the object is defective.
Known 3-D vision systems, such as the system described above, provide accurate results for measuring the dimensions of leads on chip carriers, and for coplanarity inspection of BGAs. However, there is difficulty in obtaining accurate dimensions (such as diameters) of ball, bump, and hemispherically shaped contacts, such as those found on certain chip carriers (e.g., BGAs). This difficulty is largely due to "shadow" effects. Known 3-D vision systems cannot accurately capture data concerning all portions of each ball. These systems rely on complex curve-fitting algorithms to approximate the width of each ball. Accordingly, there is a need for a simplified process for accurately determine dimensional data regarding ball, bump, and hemispherically shaped contacts.
U.S. Pat. No. 4,688,939 issued to Ray (the "'939 patent") describes a system for automatically inspecting bumps of solder on a major surface of a chip carrier. In accordance with the specification of the '939 patent, a chip carrier is placed on a platform beneath a ring light which is in registration with a television camera. Light from the ring light, which is directed at an angle towards all sides of the chip carrier, is only reflected upwardly into the television camera by solder bumps. The output signal of the television camera, which varies with the intensity of the light reflected from the solder bumps, is processed by a vision system to obtain a one-dimensional plot of the light intensity. The intensity plot is then analyzed for missing, bridged, or excessive solder bumps. This system, however, collects only one-dimensional data concerning the chip carrier. If 2- or 3-D data is desired, a separate vision system must be used.
Furthermore, the system described in the '939 patent requires that the area surrounding the solder bumps be very diffuse, serving to scatter rather then reflect light. Accordingly, measurements of solder bumps on chip carriers made of more reflective material will likely prove inaccurate.